[time-nuts] Si5451B

[John Ackermann N8UR]

On 2/3/19 11:48 AM, Achim Gratz wrote:
> Achim Gratz writes:

> Towards that end, I found a small nice looking breakout board with an
> Si5351B for around $35 (you get to solder the SMA and headers on
> yourself, the SMD stuff is already done).  Replacing the XO with a cheap
> OCXO would add another $35 it seems and I should be able to feed all
> rasPi and Tinkerboard with something a lot more stable.  Has anybody
> worked with that chip before and can share some experiences?

Hi Achim --

I've been playing with the SiLabs 53xx series chips.  There are two that 
are quite interesting for time-nuts.

The 5340/41 are pretty good, though there's a tradeoff in using them -- 
with a good reference you can get decent close-in noise, but the noise 
floor sucks.  Using a cheap 48 MHz crystal you can get a pretty good 
noise floor but the close in noise and ADEV suck.

The 5342/44/45 chips add an additional PLL using a cheap 48 MHz crystal 
as a clean-up stage.  It gives performance that's sort of like the best 
of both 5340 reference options.  These chips also allow you to adjust 
the PLL bandwidth, though you can't get as narrow as our applications 
might want.

Attached are some PN plots at 144 MHz with both the 5340 and 5342, and a 
few others for good measure.

To decode the abbreviations for the reference oscillators, "XO" is the 
cheap 48 MHz crystal on the evaluation board, "PP" is a PulsePuppy board 
with a $20 eBay Isotemp 10 MHz OCXO, and "ULN" is an expensive Wenzel 
Ultra Low Noise oscillator.  All the external oscillators gave similar 
noise performance, establishing that you don't need a super-clean 
reference to drive these chips.

I don't have plots to show it, but a higher frequency reference seems to 
have a significant impact on the noise floor -- if you can provide a 50 
MHz source with decent PN, the 5342 will be quite a bit quieter than 
with a 10 MHz reference.

John
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